This invention relates to stepped horn antennas, and particularly to stepped horn antennas usable at disparate frequencies.
Spacecraft-based communication systems often operate at disparate frequencies, as for example at 3.7-to-4.2 (3.95) GHz for downlink transmission and 5.925-to-6.425 (6.2) GHz for uplink transmission. At the spacecraft, transmission takes place at the lower frequency, and reception at the higher frequency. Because of the long transmission path lengths in satellite-based operation, and the resultant losses, it is common to use high-gain antennas at the spacecraft. Reflector-type antennas are widely used for both transmission and reception in satellite communication, because a relatively large radiating aperture can be achieved with a simple and lightweight structure. These reflector-type antennas require a feed antenna, as known in the art. Feed antennas for use with reflectors are not different from antennas used for other purposes, but their aperture distributions are tailored to produce the desired aperture distribution over the face of the reflector.
The tailoring of the aperture distribution of a reflector-type antenna by adjusting the nature of the feed antenna often requires a feed structure including a plurality of horn antennas, each of which is itself tailored to produce a portion of the aperture distribution. These several horn antennas add unwanted weight to the antenna portion of the spacecraft. As known to those involved in spacecraft, the cost of boosting or launching a mass to orbit is very great, and the on-station value of an operating communication satellite is large. Every measure is normally exerted to reduce the weight of all structures of a spacecraft, so that additional expendable propellant can be on-loaded, which allows more on-station time for the spacecraft. For this purpose, the number of reflector feed horns, and the size of each feed horn, should be kept to a minimum, commensurate with achieving appropriate radiation efficiency as measured by spillover of feed energy beyond the edges of the reflector(s).
In an antenna which uses a reflector and a plurality of feed horns to produce multiple overlapping beams on the Earth""s surface, the spacing or overlapping of the beams (the angular separation of the beams) depends, at least in part, on the spacing between feed horns. Close beam spacing, in turn, requires close spacing of the feed horns, to the point at which the horns may actually touch, at which point closer spacing is not possible. In order to achieve closer angular beam spacing, the horns themselves must be small, so that their phase centers may be placed closer together. While horn apertures can always be made smaller, small size is generally correlated with low gain and a large beamwidth. However, the large beamwidth tends to create xe2x80x9cspilloverxe2x80x9d losses, in which the feed-horn energy is not intercepted by the reflector.
In FIGS. 1a and 1b, a horn antenna 10 includes a metallic or conductive horn portion 12 defining an upper plate or wall 14u and a lower or bottom plate or wall 14b. In the embodiment of FIGS. 1a and 1b, the plates 14a and 14b extend parallel to each other, separated in a radiating-end or phasing region 16 by a left vertical plate or wall 18l and right vertical plate or wall 18r, and separated in a feed-end region 20 by a left vertical plate or wall 22l and a right vertical plate or wall 22r. The walls 14u, 14b, 18l and 18r together define a rectangular radiating aperture 26, and the walls 14u, 14b, 22l, and 22r together define a rectangular waveguide feed aperture. The direction of the electric field of the horn antenna 10 in normal operation is illustrated by arrow e, having terminations or ends at upper plate 14u and at lower plate 14b. 
Those skilled in the arts of antennas know that the term xe2x80x9cfeedxe2x80x9d and xe2x80x9cradiatingxe2x80x9d are used in respect of antennas for historic reasons rather than as accurate descriptors, since the antenna is a transducer between guided energy and unguided or radiated energy, and the transduction operates in both directions of propagation. Thus, in a transmitting mode of operation, energy to be transmitted may be applied to the feed port, and is ideally all radiated from the radiating aperture, whereas in a receiving mode of operation, unguided energy is intercepted by the xe2x80x9cradiatingxe2x80x9d aperture and is transduced to the xe2x80x9cfeedxe2x80x9d port.
As illustrated in FIGS. 1a and 1b, upper wall 14u and lower wall 14b extend from feed aperture 24 to radiating aperture 26 without a step, whereas a step in dimension exists at a plane 28 lying between radiating-end or phasing portion 16 and feed-end portion 20. A pair of vertically disposed electrically conductive walls 24l and 24r are disposed coincident with plane 28, and are in conductive contact with the ends of the vertical walls. More particularly, a vertical wall 24l is connected to that portion of wall 18l remote from radiating aperture 26 and to that portion of vertical wall 22l remote from feed aperture 24. Similarly, a vertical wall 24r is connected to that portion of wall 18r remote from radiating aperture 26 and to that portion of vertical wall 22r remote from feed aperture 24.
The specification of the electric field direction identifies the various conductive walls of metallic horn 12 as being either in the Electric (E) plane or in the magnetic (H) plane. In particular, those electrically conductive plates on which the electric field lines terminate (when they are straight) are the E-plane walls, and correspond to walls or plates 14u and 14b. Those electrically conductive walls which are parallel to straight electric field lines are designated as H plane walls. Thus, walls 18l, 22l, and 24l, and walls 18r, 22r, and 24r, are all H-plane walls.
Stepped horns are known in the art, and are described, for example, in U.S. Pat. No. 4,757,326, issued Jul. 12, 1988 in the name of Profera, Jr. As described therein, a step transition in the H-plane dimensions of the horn set up TE3,0 waveguide mode (equivalent to the LSE3,0 mode) which interacts with the principal TE1,0 mode (equivalent to the LSE1,0 mode) to linearize the electric field amplitude distribution in the radiating aperture, for thereby increasing the effective aperture in the H plane. The TE3,0 mode must be in-phase with the TE1,0 mode near the H-plane walls of the horn in order to linearize the distribution, and if it should be out-of-phase, the amplitude distribution would be such as to reduce the effective aperture of the horn. The axial length of the phasing portion 16 of the antenna 12 is selected to provide the proper phasing of the TE3,0 mode relative to the TE1,0 at the radiating aperture 26.
Improved spacecraft antennas are desired.
A horn antenna according to an aspect of the invention includes an electrically conductive first waveguide portion defining a rectangular waveguide feed aperture and a second rectangular aperture which is larger than the feed aperture, at least in the H plane. The horn includes an electrically conductive rectangular second waveguide portion defining a radiating aperture and a second aperture. The second aperture of the second waveguide portion is larger than the second aperture of the first waveguide portion in the H plane, and the second aperture of the second waveguide portion is identical in dimension to the second aperture of the first waveguide portion in the E plane. The second apertures of the first and second waveguide portions are juxtaposed with corresponding polarizations, thereby defining an H-plane step in dimension, but not an E-plane step. The horn further includes electrically conductive means or walls coupling the walls of the first and second waveguide portions at the H-plane step, to thereby define continuous H-plane walls extending from the feed to the radiating apertures. The horn also includes first and second dielectric slabs, each of which dielectric slabs lies against or is juxtaposed to the E-plane walls of the second waveguide portion, and extend from near the step to near the radiating aperture.
In a particular embodiment of the horn, at least one of the first and second portions is tapered in the E plane, and preferably both portions are tapered in the E plane. The second portion of the horn may be square in cross-section. In yet another embodiment, each of the dielectric slabs is tapered in thickness, with the thickest portion lying nearest the radiating aperture.
In another avatar of the invention, a dielectric loaded stepped horn such as that described above is used as at least a portion of a feed of a reflector-type antenna.
In yet a further manifestation, the horn may include one or more further electrically conductive walls or vanes, lying roughly parallel with the E-plane walls, and spaced away from the E-plane walls and from each other when there is more than one such further wall. The further wall or walls are physically close to the H-plane walls of the horn.